It is well known that in many engines the fuel is also the lubricant for the fuel system components, such as fuel pumps and injectors. Many studies of fuels with poor lubricity have been conducted in an effort to understand fuel compositions that have poor lubricity and to correlate lab test methods with actual field use. The problem is general to diesel fuels, kerosene and gasolines, however, most of the studies have concentrated on the first two hydrocarbons.
Previous work has shown that saturated, monomeric and dimeric, fatty acids of from 12 to 54 carbon atoms used individually give excellent performance as fuel lubricity aids in diesel fuels. Fatty acids are for the most part unbranched. A number of other kinds of lubricity additives are also known. Since the advent of low sulfur diesel fuels in the early 1990s, relatively large amounts of these lubricity additives have been used to provide a fuel that does not cause excessive wear of engine parts.
Unfortunately, many commercially available fatty acids and fatty acid blends tend to freeze or form crystals at temperatures common during winter weather. The freezing or formation of crystals makes handling of the additives, and particularly injection into fuel, difficult. Blending the fatty acid with a solvent can lower the freezing point and reduce the crystal formation temperature, or cloud point. However, addition of a solvent may increase cost and preparation complexity.
Some of the fatty acids, fatty acid ammonium salts and fatty acid amides presently used may have the disadvantage of solidifying on storage at low temperatures. Often even at room temperature, crystalline fractions may separate and cause handling problems. Diluting the additives with organic solvents only partly solves the problem, since fractions may still crystallize out from solutions or the solution may gel and solidify. Thus, for use as lubricity additives, the fatty acids, fatty acid ammonium salts and fatty acid amides either have to be greatly diluted or kept in heated storage vessels and added via heated pipework.
It is also known to make fuel lubricity additives by reacting alkylene glycols with monomeric and dimeric carboxylic acids using sulfur-containing catalysts such as toluene sulfonic acid. However, this method takes considerable time, on the order of 40 to 60 hours, produces significant amounts of alkylene glycol and water as by-products and leaves certain amounts of sulfur in the additive which undesirably wind up in the hydrocarbon fuel. Further, as environmental regulations have become more stringent, low sulfur fuels are mandated to have increasingly lower amounts of sulfur present and even the small amounts present in these additives become problematic.
Thus, it would be desirable if a way could be discovered to enhance the lubricity of distillate fuels, but that involved essentially no sulfur. Further, it would be helpful if a no-sulfur fuel additive could be made by a process relatively quickly without large amounts of by-products.